Why do I get an ECONNREFUSED - Connection refused by server error when connecting proftpd?

Question

ProFTPD

After connecting to FTP for a while everything works fine. But then the connection is terminated, the port is unavailable, and an error appears
Failed to establish a connection with "ECONNREFUSED - Connection refused by server".
after restarting proftpd, it works again for a while. What could be the problem? How to detect and solve it?

Answer 1

well, first look at the proftpd logs, by default in Linux it is here:
Then you can see the output of dmesg.

Answer 2

Logs

2015-03-29 07:12:26,004 vps-13799.vps-ukraine.com.ua proftpd[24837] localhost (91.212.124.39[91.212.124.39]): USER user: Login successful. 
2015-03-29 07:13:05,612 vps-13799.vps-ukraine.com.ua proftpd[24772] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:13:05,612 vps-13799.vps-ukraine.com.ua proftpd[24772] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 08:13:07,431 vps-13799.vps-ukraine.com.ua proftpd[24905] localhost (91.212.124.39[91.212.124.39]): FTP session opened. 
2015-03-29 07:13:07,547 vps-13799.vps-ukraine.com.ua proftpd[24905] localhost (91.212.124.39[91.212.124.39]): Preparing to chroot to directory '/home/user' 
2015-03-29 07:13:07,548 vps-13799.vps-ukraine.com.ua proftpd[24905] localhost (91.212.124.39[91.212.124.39]): USER user: Login successful. 
2015-03-29 07:16:59,043 vps-13799.vps-ukraine.com.ua proftpd[24036] localhost (91.212.124.39[91.212.124.39]): FTP no transfer timeout, disconnected 
2015-03-29 07:16:59,050 vps-13799.vps-ukraine.com.ua proftpd[24036] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:16:59,052 vps-13799.vps-ukraine.com.ua proftpd[24036] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 07:19:17,285 vps-13799.vps-ukraine.com.ua proftpd[24037] localhost (91.212.124.39[91.212.124.39]): Passive data transfer failed, possibly due to network issues 
2015-03-29 07:19:17,286 vps-13799.vps-ukraine.com.ua proftpd[24037] localhost (91.212.124.39[91.212.124.39]): Check your PassivePorts and MasqueradeAddress settings, 
2015-03-29 07:19:17,286 vps-13799.vps-ukraine.com.ua proftpd[24037] localhost (91.212.124.39[91.212.124.39]): and any router, NAT, and firewall rules in the network path. 
2015-03-29 07:19:17,286 vps-13799.vps-ukraine.com.ua proftpd[24037] localhost (91.212.124.39[91.212.124.39]): FTP no transfer timeout, disconnected 
2015-03-29 07:19:17,292 vps-13799.vps-ukraine.com.ua proftpd[24037] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:19:17,293 vps-13799.vps-ukraine.com.ua proftpd[24037] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 07:19:18,151 vps-13799.vps-ukraine.com.ua proftpd[24355] localhost (91.212.124.39[91.212.124.39]): Data transfer stall timeout: 600 seconds 
2015-03-29 07:19:18,154 vps-13799.vps-ukraine.com.ua proftpd[24355] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:19:18,154 vps-13799.vps-ukraine.com.ua proftpd[24355] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 07:20:23,610 vps-13799.vps-ukraine.com.ua proftpd[24477] localhost (91.212.124.39[91.212.124.39]): Data transfer stall timeout: 600 seconds 
2015-03-29 07:20:23,613 vps-13799.vps-ukraine.com.ua proftpd[24477] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:20:23,615 vps-13799.vps-ukraine.com.ua proftpd[24477] localhost (91.212.124.39[91.212.124.39]): notice: user user: aborting transfer: Data connection closed 
2015-03-29 07:20:23,616 vps-13799.vps-ukraine.com.ua proftpd[24477] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 07:20:23,678 vps-13799.vps-ukraine.com.ua proftpd[24427] localhost (91.212.124.39[91.212.124.39]): Data transfer stall timeout: 600 seconds 
2015-03-29 07:20:23,683 vps-13799.vps-ukraine.com.ua proftpd[24427] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:20:23,684 vps-13799.vps-ukraine.com.ua proftpd[24427] localhost (91.212.124.39[91.212.124.39]): notice: user user: aborting transfer: Data connection closed 
2015-03-29 07:20:23,684 vps-13799.vps-ukraine.com.ua proftpd[24427] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 08:43:40,866 vps-13799.vps-ukraine.com.ua proftpd[26257] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): FTP session opened. 
2015-03-29 08:43:40,866 vps-13799.vps-ukraine.com.ua proftpd[26258] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): FTP session opened. 
2015-03-29 07:43:41,073 vps-13799.vps-ukraine.com.ua proftpd[26258] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): Preparing to chroot to directory '/home/user' 
2015-03-29 07:43:41,073 vps-13799.vps-ukraine.com.ua proftpd[26257] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): Preparing to chroot to directory '/home/user' 
2015-03-29 07:43:41,073 vps-13799.vps-ukraine.com.ua proftpd[26258] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): USER user: Login successful. 
2015-03-29 07:43:41,073 vps-13799.vps-ukraine.com.ua proftpd[26257] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): USER user: Login successful. 
2015-03-29 07:52:04,608 vps-13799.vps-ukraine.com.ua proftpd[24837] localhost (91.212.124.39[91.212.124.39]): Data transfer stall timeout: 600 seconds 
2015-03-29 07:52:04,617 vps-13799.vps-ukraine.com.ua proftpd[24837] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:52:04,619 vps-13799.vps-ukraine.com.ua proftpd[24837] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 07:52:04,631 vps-13799.vps-ukraine.com.ua proftpd[24905] localhost (91.212.124.39[91.212.124.39]): Data transfer stall timeout: 600 seconds 
2015-03-29 07:52:04,635 vps-13799.vps-ukraine.com.ua proftpd[24905] localhost (91.212.124.39[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 07:52:04,635 vps-13799.vps-ukraine.com.ua proftpd[24905] localhost (91.212.124.39[91.212.124.39]): FTP session closed. 
2015-03-29 08:55:55,202 vps-13799.vps-ukraine.com.ua proftpd[26658] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): FTP session opened. 
2015-03-29 07:55:55,608 vps-13799.vps-ukraine.com.ua proftpd[26658] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): Preparing to chroot to directory '/home/user' 
2015-03-29 07:55:55,609 vps-13799.vps-ukraine.com.ua proftpd[26658] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): USER user: Login successful. 
2015-03-29 08:00:08,515 vps-13799.vps-ukraine.com.ua proftpd[26658] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): ROOT PRIVS: unable to seteuid(): Operation not permitted 
2015-03-29 08:00:08,515 vps-13799.vps-ukraine.com.ua proftpd[26257] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): ROOT PRIVS: unable to seteuid(): Operation not permitted 
2015-03-29 08:00:08,515 vps-13799.vps-ukraine.com.ua proftpd[26257] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): ROOT PRIVS: unable to setegid(): Operation not permitted 
2015-03-29 08:00:08,515 vps-13799.vps-ukraine.com.ua proftpd[26658] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): ROOT PRIVS: unable to setegid(): Operation not permitted 
2015-03-29 08:00:08,522 vps-13799.vps-ukraine.com.ua proftpd[26257] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 08:00:08,522 vps-13799.vps-ukraine.com.ua proftpd[26258] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): ROOT PRIVS: unable to seteuid(): Operation not permitted 
2015-03-29 08:00:08,522 vps-13799.vps-ukraine.com.ua proftpd[26258] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): ROOT PRIVS: unable to setegid(): Operation not permitted 
2015-03-29 08:00:08,522 vps-13799.vps-ukraine.com.ua proftpd[26257] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): FTP session closed. 
2015-03-29 08:00:08,523 vps-13799.vps-ukraine.com.ua proftpd[26658] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 08:00:08,523 vps-13799.vps-ukraine.com.ua proftpd[26658] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): FTP session closed. 
2015-03-29 08:00:08,525 vps-13799.vps-ukraine.com.ua proftpd[26258] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): RELINQUISH PRIVS: unable to seteuid(PR_ROOT_UID): Operation not permitted 
2015-03-29 08:00:08,525 vps-13799.vps-ukraine.com.ua proftpd[26258] localhost (109-162-17-52-klsh.broadband.kyivstar.net[91.212.124.39]): FTP session closed.